1. Field of the Invention
The present invention relates to a motor driving device that converts AC (alternating current) electric power into DC (direct current) electric power and then converts it into AC electric power again to control motors driven by the AC electric power, and includes a maximum output calculation unit of a DC conversion unit.
2. Description of the Related Art
In a machine tool system, each drive axis of a machine tool is provided with a motor unit and the motor units are driven by a motor driving device. The motor driving device commands and controls positions, velocities or torques of rotation of the motor units for driving axes of the machine tool, wherein the number of motor units is same as the number of the control axles of the machine tool.
The motor driving device includes: a DC conversion unit that converts three-phase commercial AC electric power into DC electric power; and AC convertion units that convert the DC electric power output from the DC conversion unit into AC electric power of a desired frequency supplied as driving electric power of the motor units, and controls position, velocity or torque of the motor unit connected to the corresponding AC conversion unit. The number of the AC conversion units is same as the number of the motor units so that the AC conversion units supplies driving electric power independently to the motor units provided corresponding to a plurality of driving axes in the machine tool. On the other hand, the single DC conversion unit is provided with respect to the plurality of the AC conversion units to reduce cost and space occupation of the motor driving device. Thus, in the motor driving device, the single DC conversion unit supplies the DC electric power to the plurality of the AC conversion units to reduce cost and space occupation in comparison with the case in which a plurality of DC conversion units are provided.
In the motor driving device configured as described above, in order to stably and surely drive the plurality of motor units, sufficient driving electric power has to be supplied to each of the plurality of motor units. To that end, the DC conversion unit has to be selected so that, when the DC conversion unit converts three-phase AC input into DC, its maximum output is larger than the summation of the maximum outputs of the plurality of the motor units. Conventionally, from the value obtained by simply summing the maximum outputs of the plurality of the motor units, the maximum output of the DC conversion unit that converts three-phase AC into DC is determined.
Japanese Unexamined Patent Specification No. 2010-74918 describes an invention in which total electric power consumption of a plurality of AC converting units or regenerated electric power that is regenerated from the AC converting units to a commercial electric power supply is calculated to calculate electric power consumption in a motor driving device.
Depending on operating conditions and circumstances of the machine tool, all of the motor units may not be driven at the maximum output at the same time. In this case, in the motor driving device, all the AC conversion units do not have to output the driving electric power to drive all the motor units at the maximum output. As a result, the DC conversion unit does not have to output the DC electric power larger than the necessary electric power assuming the conditions described above. It is therefore advantageous in view of cost and space occupation to estimate the total electric power actually consumed by all of the motor units and select a DC conversion unit from a plurality of DC conversion units having the maximum output that is at least necessary and that is larger than the estimated total electric power to some extent as an “optimal” DC conversion unit.
However, in order to select the DC conversion unit from a plurality of DC conversion units, a user or designer of a machine tool has to measure input current and input voltage values one by one by using measuring instruments and calculate the electric power consumption values of the motor units individually by using various motor constants when the machine tool actually operates under desired operating conditions and circumstances. Then, in consideration of the calculation results, the user or designer has to estimate the total electric power consumption of the AC conversion units. Thus, it takes a lot of time and effort to select the DC conversion unit from a plurality of DC conversion units. Further, even if the selection of the DC conversion unit is made based on the total electric power consumption of the AC conversion units estimated as described above, the optimal DC conversion unit is not always selected.
Further, in the invention described in Japanese Unexamined Patent Specification No. 2010-74918, the total electric power consumption of the plurality of AC conversion units in the motor driving device and electric power regenerated to the commercial electric power supply are calculated collectively. However, the operating conditions or circumstances under which the individual motor units and AC conversion units may operate are not considered. As a result, even if this invention is used, the optimal DC conversion unit may not be selected.